In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed.
As electronic devices in which a thin film transistor is used, there are mobile devices such as a mobile phone or a notebook computer, and the like. For such a portable electronic device, power consumption which affects continuous operating time is a big problem. Also for a television or the like set which is increasing in size, it is important to suppress increase in power consumption associated with the increase in size.
In addition, in a display device, when image data which is input to a pixel is rewritten, an operation of writing the same image data is performed once again even in the case where image data in a period is the same as that in the preceding period. As a result, by performing the operation of writing the same image data a plurality of times, power consumption is increased. In order to suppress such increase in power consumption in a display device, for example, a technique has been disclosed in which a break period which is longer than a scanning period is set as a non-scanning period every time after image data is written by scanning a screen in the case of displaying a still image (for example, see Patent Document 1 and Non-Patent Document 1).